1. Field of the Invention
The present invention relates to an optical encoder having a combination of a uniform grating and a nonuniform grating.
2. Description of the Prior Art
A conventional optical encoder includes a main scale and an index scale which are arranged in parallel with each other so as to be relatively movable, a light source for irradiating collimated light on the scales and a photo detector for detecting an shading pattern modulated by the overlap state between the main scale and the index scale. Each of the main scale and the index scale has an optical grating. Generally, the optical gratings of the scales are composed of transmitting portions and non-transmitting portions which are alternately arranged with a predetermined width of 1:1 at a predetermined pitch. Assuming that there is no diffraction, when the collimated light is irradiated to each of the scales, the light amount distribution transmitted through the grating of each of the scales shows a rectangular wave pattern. Hereinafter, such a grating is referred to as a "rectangular wave grating".
When the main scale and the index scale are relatively displaced, the output light quantity obtained in accordance with the overlap condition of such rectangular wave gratings ideally becomes a triangular wave signal. In fact, however, it becomes a pseudo-sine curve because of diffraction and so on. A relative position is detected by using the sine wave output signal as a displacement signal. Generally, a pair of index scales is used to obtain so-called A phase and B phase sine wave signals which are mutually phase shifted of 90 degrees.
The above mentioned displacement signal formed of a pseudo-sine wave is different from an ideal sine wave signal. Namely, a waveform distortion is large. Especially, the distortion rate depends on a fluctuation of the gap between the main scale and the index scale. The fluctuation of the distortion rate is mainly caused by odd ( third, fifth, . . . ) harmonic components included in the displacement signal. If position measurement is performed by use of the displacement signal including the distortion rate fluctuation, measurement errors get large.
Several technologies to reduce the measurement errors accompanied with the waveform distortion in the displacement signals have been suggested. For example, it is known that two rectangular grating patterns are provided on a scale, the grating patterns being phase shifted a little each other, to cancel the harmonic components in the displacement signal by adding two output signals(refer to U.S. Pat. No. 5,068, 530 ).
Another prior art suggests that a combination of gratings, one of which is formed of rectangular patterns and the other is formed of sine wave patterns, can prevent output signal from including the harmonic components refer to U.S. Pat. No. 4,782,229 ). Hereinafter, a grating having sine wave patterns, which can fundamentally obtain only sine wave outputs by combination with a rectangular wave grating, is referred to as a "sine wave grating".
By the above mentioned conventional technologies, it is possible to reduce the wave distortion factor roughly. However, it is not yet possible to obtain a precision highly enough to apply for a position measuring in a precise machine manufacture and so on. Further, it is difficult to make a sine wave grating pattern in view of a current manufacturing technology.